When drilling in subsea applications, which can be at a water depth of as much as 10,000 feet (3,000 metres), it is important to know the location of the equipment with respect to the BOP, the wellhead, in the cased hole and in the bore of the drilled well. For example, it is important to know how equipment needs to be positioned in and along the bore for operations to be performed correctly.
The prime operations are: drilling the well, casing and cementing, well testing, completion and running any equipment inside the completion, a well workover and well intervention. In addition to the well operations, there are the system tests to check the integrity of individual systems and that they are performed as required. These may include the well, wellhead and BOP pressure tests and the BOP operating tests. A subsea well also creates additional complications in respect to a well kick operation or underbalance drilling (i.e. snubbing in or out of the hole) and the requirement to carry out an emergency disconnect and later the reestablishment of the well.
While carrying out all these operations from a floating vessel, it is important to know accurately, at any instant, the position of items of equipment within the system.
When drilling a subsea well, the prime pressure containing equipment that contains possible formation pressures includes the subsea wellhead, the casing which is hung from and cemented to the wellhead and the BOP on the wellhead.
A BOP assembly is a multi closure safety device which is connected to the top of a drilled, and often partially cased, hole. The accessible top end of the casing is terminated using a casing spool or wellhead housing to which the BOP assembly is connected and sealed.
The wellhead and BOP stack (the section in which rams are provided) must be able to contain fluids at a pressure rating in excess of any formation pressures that are anticipated when drilling or when having to pump into the well to suppress or circulate an uncontrolled pressurized influx of formation fluid. This influx of formation fluid is known as a “kick” and reestablishing control of the well by pumping to suppress the influx or to circulate the influx out under pressure is known as “killing the well”. An uncontrolled escape of fluid, whether liquid or gas, to the environment is termed a ‘blow out’. A blow out can result in major leak to the environment which can ignite or explode, jeopardizing personnel and equipment in the vicinity, and pollution.
Although normal drilling practices provide a liquid hydrostatic pressure barrier to a kick, a final second safety barrier is provided mechanically by the BOP assembly. The BOP assembly must close and seal on tubular equipment (i.e., pipe, casing or tubing) hung or operated through the BOP assembly and ultimately must be capable of shearing and sealing off the well. A general term for a tubular system run into the well is called a string. Wells are typically drilled using a tapered drill string having successively larger diameter of tubulars at the lower end. When running a completion or carrying out a workover, various diameter of tubulars, coiled tubing, cable and wireline and an assortment of tools are run. In addition, dual tubulars, or tubulars with pipes and cables as a bundle, must be considered.
A subsea conventional BOP assembly is attached to a wellhead and is provided with a number of rams either to seal around different set tubular diameters or to shear and seal the bore. These rams should be rated to perform at pressures in excess of any anticipated well pressures or kick control injection pressures which are approximately 10 to 15 kpsi (69-103 MPa). A minimum of one annular is provided above the rams to cater for any tubular diameter or for stripping in or out under pressure. An annular is a hydraulically energized elastometric toroidal unit that closes and seal on varying diameters of tubular member whether stationary or moving into or out of the well. Due to the nature of this pressure barrier element, a lower maximum rated working pressure of about 5 kpsi (34 MPa) is normally available.
Above the annulars, there are no further well pressure barrier elements with the riser only providing a hydrostatic head, liquid containment and guidance of equipment on a normal pressure controlled drilling operation. For a subsea riser system, the hydrostatic head of the different drilling liquids over the ambient sea water pressure means the low pressure zone above the subsea BOP assembly must still withstand hydrostatic pressures of, depending upon the depth of water, approximately 5 kpsi (34 MPa).
The conventional BOP assembly in effect provides a three zone pressure containment safety system. The three zones typically consists of the first high pressure lowermost section encompassing the rams, the medium pressure second zone having the annular or annulars and the low pressure third zone being the bore open to atmosphere and, on a subsea system, the riser bore to the surface vessel. Therefore it is critical that the correct rams are closed on the correct diameter and full pressure integrity is achieved. In an emergency disconnect it is important that, besides sealing on the pipe or tubular, the pipe is held and not dropped down the hole.
A BOP can be fitted with a ram or rams to suit various diameters of drill pipe, tubing or casing. Variable rams can be used, having carefully selected their range. A BOP is fitted with the rams mostly likely to be needed in a certain drilling/workover phase. If a stage is reached where an inadequate range of rams are in the BOP to handle the tools/equipment to be used in the next sequence, the BOP has to be pulled and appropriately redressed.
When drilling or carrying out well intervention on a subsea well where the wellhead is at the seabed, the subsea BOP attached to the subsea wellhead is connected to a buoyant floating drilling vessel by a riser. A floating drilling vessel should maintain its station vertically above the well to enable well operations to be performed.
Failure to do so caused by weather conditions, current forces, equipment malfunctions, drift off or drive off, fire or explosion, collision of other marine incidents means it is necessary if possible to make the well safe, isolate the well at the seabed and disconnect the riser system. In a severe emergency, shearing any tubulars or equipment in the BOP bore, sealing the well to full working pressure and disconnecting the riser system is required to be achieved in under 30 seconds.
At present, in order to know what components are run through the drill floor, a manual record of the relevant dimensions, such as the length and the diameter of components are logged. These records are typically made in a notebook before being totalled up. Mathematical errors can occur easily during the totaling or components can be left out of the tally entirely or additional equipment, over and above that scheduled to be run, run in through the rotary table can be ignored or forgotten. Therefore, on a number of occasions, the accuracy of the tally is questionable.
Furthermore, as there are a wide variety of components which can be run in the hole, often with minor variations in length for what otherwise appear to be identical components, it is important that each component is measured individually before it is attached to the string. It is easy for minor errors in measurement of each component to add up to a significant error over the length of the string.
A further problem is that even when the measurements are accurately taken at the rig, these measurements are passive, i.e. on unstressed dimensions of the component. Once the component has been run in on a string, it may have 5,000 metres of additional components hanging from its end and, although this would not produce a significant change in length of a single component, when the total change is added-up over all components of the drill string, the change can be significant.
Furthermore, as the riser extending between the wellhead and the drill rig may be 2000-3000 m in length, it is subject to subsea currents and may be caused to “snake” between the rig and the wellhead. In this case, the length of drill string run into the riser is not directly comparable to the straight distance between the rig and the wellhead.
Additional problems are encountered as the drilling rig heaves on the sea surface such that its position, which is dependent on the tide and the vessel draft, is constantly changing with respect to the sea bed. This can, in part, be compensated for by the use of telescopic joints and a travelling block, but these additional factors need also to be included in any calculation of the position of the string. As the rig can heave in a matter of seconds, it can, in rough conditions, be impossible to determine accurately the position of the string given that the calculations required at present are cumbersome and complex.
It is critical at certain instances to know the position of equipment in the hole and, on a floating vessel, this requires knowledge of the tally, water depth, the draft and any change of draft of the vessel, swell or tidal heave, position of the travelling block, the stroke of the compensator and the depth of hole drilled since the last summation was made. This does not take into account snaking of the riser due to currents or cross currents in deep water, or the extension of the tubular string due to tension and weight. It is therefore difficult to determine accurately what component is at any given depth in a quick and accurate manner.
An example outlining a subsea well operation is an emergency disconnect involving the drilling string.
The accurate position of the drill string is required in the event of an emergency shut in of the BOP by closure of, for example, the shear blind rams in the BOP stack. The shear blind rams are those which can cut the drill string or a pipe or tubing and then seal the BOP bore when there is a need to carry out an emergency disconnect of the riser system from the BOP stack. The shear blind rams are activated with only a set force and therefore, should the rams close on a section of equipment which is significantly larger than the shear capability of the rams, for example on a joint between adjacent pipe sections, the rams may not fully sever the drill string thereby not closing sufficiently to seal the well and allow an emergency disconnect to be carried out correctly. To prevent the drill string falling down the hole, and to enable the drill string to be available to kill and circulate the well on reconnection, it is very advisable to be able to hang the drill string off on a set of pipe rams. This is achieved by resting an up-set diameter of the string on a set of pipe rams below the blind shear rams.
For operations of this sort, it is necessary to know the position of a specific part of the drill string to approximately one metre over anything up to 3,000 metres:
Further examples in which it is important to know the precise position of equipment is in testing of the BOP, testing the wellhead, flow testing the well, kick control, well circulation and testing of spool trees between the wellhead and the BOP.
Accordingly, it is an aim of the present invention to provide a system which enables the above problems to be overcome and allows the operator of the drilling system to know the precise position of a string, which may be moving, relative to a section of the well, the BOP or the wellhead at any given moment.